(a) Field of the Invention
The present invention relates to a plasma display panel (PDP). More specifically, the present invention relates to a method and device for displaying grays of a PDP that can reduce contour noise occurring when displaying moving pictures on the PDP.
(b) Description of the Related Art
A PDP arranges a plurality of discharge cells in a matrix format and emits light selectively using the discharge cells. Accordingly, the PDP is a display that restores input image data using electric signals.
In order to provide a fully functional color display PDP, the PDP is required to display different gray levels. A method of splitting a single field into a plurality of sub-fields and controlling the time division is used to implement different gray levels.
FIG. 1 shows a gray display method used in a conventional AC surface discharge PDP. In the drawing, the horizontal axis represents time and the vertical axis indicates a number of horizontal scanning lines.
This 8-bit gray implementation method divides a single field into eight sub-fields, and each sub-field includes an addressing period and a discharge-sustain period. The addressing period forms a wall charge on a selected cell of the PDP using a selective discharge by a write pulse, and writes relevent information. The discharge-sustain period represents a light-emitting period that actually displays images on the actual screen through discharging by continuous discharge-sustain pulses.
The discharge-sustain period has a light-emitting period ratio of 1:2:4:8:16:32:64:128. The different gray level on the PDP is implemented by accumulating in the human eyes for a period of time the lights irradiated by the selectively flickering sub-fields and have persons perceive the gray levels of average luminance
For example, so as to implement the gray of level 3, when a sub-field having a period of 1T and a sub-field having a period of 2T are flickered, and a summation of the flickering period becomes 3T, human eyes perceive the gray of level 3 displayed with the disposed beams during the period of 3T. Similarly, when the gray of level 127, sequentially flickering the sub-fields respectively having the periods of 1T, 2T, 4T, 8T, 16T, 32T and 64T can achieve the luminance of level 127 according to the beams volume exposed during the total period of 127T. Thus, eight sub-fields can display all 256 grays (28=256).
The above-described PDP gray driving method works great for still images. When a viewer's viewpoint moves, however, the viewer see a distorted image displayed on the PDP. This is calles as contour noise, Contour noise depends on a product of light-emitting time of a pixel and a moving speed of a point and time asynchronism of the light-emitting. As a result, it distorts the gray levels or colors.
In particular, when the gray levels 127 and 128 are adjacent to each other, beams are emitted during the period of 1T, 2T, 4T, 8T, 16T, 32T, and 64T, which are provided on a first portion of a single field with respect to time so as to display the gray level 127. Then, beams are emitted during the period of 128T provided on a second portion of the single field. Accordingly, these two cases have a big difference of light-emitting time locations in the single field, generating a very high contour noise.
FIG. 2 shows a screen on which a contour noise is generated on a conventional AC-type surface discharge PDP.
As shown, the gray level of the left four columns of pixels is 127, and the gray level of the right four columns of the pixels is 128. When the pattern moves in the left direction by one pixel pitch for each field, a bright dispersion is formed on a boundary line A where the pixels of gray level 127 and the pixels of gray level 128 meet.
Conventional methods for reducing the contour noise are disclosed in the Japanese laid-open patents Nos. 1999-327491 and 1999-73157. In the first Japanese laid-open patent No. 1999-327491, each field is divided into light-emitting periods and pulse blanking periods with respect to time, and each light-emitting period is divided into a plurality of sub-fields with respect to time. In the pulse blanking period, neither a scanning electrode pulse nor a sustain electrode pulse is supplied, and scanning electrode and sustain electrode voltages are sustained at a predetermined level. Also, in the Japanese laid-open patent No. 1999-73157, the PDP includes a plurality of row electrodes corresponding to display lines and arranged in the horizontal direction, and a plurality of column electrodes arranged in the direction vertical to the row electrodes and forms cells where each row electrode and column electrode crosses. The display period is divided into a plurality of division periods, and a plurality of light-emitting modes. Each of the light-emitting modes has an order of a light-emitting period corresponding to the division period. Such light-emitting modes are alternately performed for each discharge cell or a block of discharge cells where a plurality of adjacent discharge cells form couples.
However, since the above-described conventional methods cannot sufficiently reduce the contour noise displayed longitudinally on the screen, viewers can recognize it.